Study of ion and vacuum ultraviolet-induced effects on styrene- and ester-based polymers exposed to argon plasma

Plasma-polymer interactions are important for the purpose of etching, deposition, and surface modification in a wide range of different fields. An Ar discharge from an inductively coupled plasma reactor was used to determine the factors in a simple plasma that control etch and surface roughness behavior for three styrene-based and three ester-based model polymers. The authors compared the etch behavior of polymers in Ar plasma discharges with low and high energy ions by changing the substrate bias, compared cooled and elevated substrate temperature conditions, and compared fully plasma-exposed conditions and vacuum ultraviolet (vuv)-only conditions by employing a magnesium fluoride window to prevent ion bombardment in the vuv-only case. It was found that ions, vuv radiation, and temperature all had significant impact on the etch behavior of polymers. The dependence of polymer structure on etch and surface roughness was also compared. Polymers with styrene and ester side groups were compared and polymers with alpha-hydrogen and with alpha-methyl were compared. It was found that for styrene-based polymers, there was a large difference in material removal between alpha-hydrogen [poly(4-methylstyrene)(P4MS)] and alpha-methyl [poly(alpha-methylstyrene) (P alpha MS)] structures. This difference was highly temperature dependent, and the ceiling temperature of the polymers was found to be the most important property to consider. Below the ceiling temperature, the amount of material removed in P4MS and P alpha MS was the same, but above it there was a dramatic material loss in P alpha MS not seen in P4MS. For the ester-based polymers, it was established that oxygen depletion occurred before any other mechanism and the most important factor to consider was oxygen content in the polymer. By using in situ ellipsometry, it was also found that at temperatures below the ceiling temperature modification by vuv radiation of P alpha MS created a slightly denser layer at the surface with higher index of refraction. This effect was not seen in P4MS. It was observed that in general, low energy ions contributed to material removal by physical sputtering at the polymer surface and the amount of material removal increased with oxygen content in the polymer. vuv radiation caused bulk depolymerization and oxygen depletion reactions that were highly polymer structure specific and temperature dependent. High energy ion bombardment was found to create an amorphous carbonlike damage layer with a thickness that was determined by the ion penetration depth. This damage layer could be characterized by ellipsometry. While for P4MS it was sufficient to model by ellipsometry the etch process using an ion-damaged layer on top of a bulk layer of unmodified polymer, the vuv effect needed to be added to the optical model in order to accurately characterize P alpha MS. Finally, surface roughening of polymers only occurred under ion bombardment. High energy ion bombardment produced the greatest roughness and corresponded to densification of the ion-damaged layer at the surface. Polymers that exhibited greater material loss to create the damaged layer showed the highest roughness.